chapter 3

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Chapter 3

Variables and Calculations

Copyright © 2011 Pearson Addison-Wesley

The TextBox Control

• A text box is a rectangular area on a form that accepts input from a keyboard

Chapter 3- Slide 2


Public Class Form1 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e

As System.EventArgs) Handles Button1.Click Label2.Text = "hello " & TextBox1.Text End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

Me.Close() End SubEnd Class

Chapter 3- Slide 3


Using the Text Property in Code

• The TextBox control’s Text property can be accessed in code the same way you access other properties

• For Example:– The contents of the Text property can be assigned into

a Label control’s Text property:– lblInfo.Text = txtInput.Text– The contents of the Text property can be displayed in a

message box– MessageBox.Show(txtInput.Text)

Chapter 3- Slide 4


Clearing a Text Box

• Can be done with an assignment statement:– txtInput.Text = String.Empty– OR txtInput.Text = “”– assigning the predefined constant String.Empty replaces

whatever text was in txtInput with an empty string• Can also be done with a method:

– txtInput.Clear()– Clear is a Method, not a Property– Methods are actions – as in clearing the text– Uses the form Object.Method()

Chapter 3- Slide 5


String Concatenation

• Assume the user has entered their name into the TextBox txtName

• Label lblGreeting can say, “Hello” to any name found in the TextBox– lblGreeting.Text = "Hello " & txtName.Text– Appends user name in txtName.Text to “Hello ”

and stores result in text property of lblGreeting

Chapter 3- Slide 6


String Concatenation

• another example of how to concatenate strings from text boxes

Chapter 3- Slide 7

txtDayOfWeektxtMonthtxtDayOfMonthtxtYearlblDateString

btnExitbtnClearbtnShowDate


The Focus Method

• For a control to have the focus means that it is ready to receive the user's input

• In a running form, one and only one of the controls on the form may have the focus

• The focus can be set to a control in code using the Focus method:

txtUserName.Focus()

Chapter 3- Slide 9


The Focus Method

• You can tell which control has focus by its characteristics:– When a TextBox has focus, it will have a blinking

cursor or its text will be highlighted– When a button, radio button, or a check box has

focus, you’ll see a thin dotted line around the control

Chapter 3- Slide 10


Assigning Keyboard Access Keys to Buttons

• Say your form had a button with the text “Exit" on it

• You can allow the user to activate the button using Alt-X instead of a mouse click

• Just change the button text property to “E&xit"• The character following the '&' (x in this case) is

designated as an access key• Be careful not to use the same access key for two

different buttons

Chapter 3- Slide 11


'&' Has Special Meaning in a Button

• Note that the '&' in “E&xit" does not display in the button control on the form

• It simply establishes the Alt Key access

• In order to actually display an '&' on a button, it must be entered as "&&“– Button text Save & Exit is

entered as Save && Exit

Chapter 3- Slide 12




Section 3.2

VARIABLES AND DATA TYPES

Variables hold data that may be manipulated, used to manipulateother data, or remembered for later use.


Declaring Variables

• A variable declaration is a statement that creates a variable in memory• The syntax is:

Dim VariableName As DataType– Dim (short for Dimension) is a keyword– VariableName is the programmer designated name– As is a keyword– DataType is one of many possible keywords for the type of value

the variable will contain

• Here is an example of a variable declaration:

Dim intLength as Integer

Chapter 3- Slide 14


Declaring Multiple Variables

• Several variables may be declared in one statement if they all hold the same type of value

Dim intLength, intWidth, intHeight as Integer

• Or this can be done in 3 separate statementsDim intLength as IntegerDim intWidth as IntegerDim intHeight as Integer

Chapter 3- Slide 15


Setting the Value of a Variable

• An assignment statement is used to set the value of a variable, as in:– Assign the value 112 to the variable length– intLength= 112– Assign the string literal “Good Morning “ followed

by the contents of the text box txtName to the variable greeting

– greeting = "Good Morning " & txtName.Text

Chapter 3- Slide 16


Visual Basic Data Types

• Integer types– Byte– Short– Integer– Long

• Floating-Point types– Single– Double– Decimal

• Other data types

– Boolean– Char– String– Date

Chapter 3- Slide 17


The String Data Type

• A string literal is enclosed in quotation marks– The following code assigns the name Jose Gonzales to

the variable strNameDim strName as stringstrName = "Jose Gonzales"

• An empty string literal can be coded as:– Two consecutive quotation marks

strName = ""– Or by the special identifier String.Empty

strName = String.Empty

Chapter 3- Slide 18


The Date Data Type

• Date data type variables can hold the date and time or both– You can assign a date literal to a Date variable, as shown

here:Dim dtmBirth As DatedtmBirth = #5/1/2010#

• A date literal is enclosed within # symbols– All of the following Date literals are valid:

#12/10/2010##8:45:00 PM##10/20/2010 6:30:00 AM#

Chapter 3- Slide 19


Default Values and Initialization

• When a variable is first created in memory, it is assigned a default value– numeric types are given a value of zero– Boolean types are given a value of False– strings are given a value of Nothing– dates default to 12:00:00 AM January 1,1

• Good practice to initialize string variables– Dim strName as String = String.Empty– String with value Nothing causes error if used

Chapter 3- Slide 20


Initialization of Variables

• Can provide a starting or initialization value for any type of variable in a Dim statement

• Usually want to set an initial value unless assigning a value prior to using the variable

• Just append = value to the Dim statement where value is the literal to be assigned to the variable

Dim intMonthsPerYear As Integer = 12

Chapter 3- Slide 21


Scope and Local Variables

• Scope refers to the part of the program where:– A variable is visible and– May be accessed by program code

• Variables declared within a procedure are called local variables and observe these characteristics– Scope begins where variable is declared– Extends to end of procedure where declared– Variable is not visible outside the procedure

• A variable cannot be declared twice in the same procedure

Chapter 3- Slide 22




Section 3.3

PERFORMING CALCULATIONS

Visual Basic has powerful arithmetic operators that perform calculationswith numeric variables and literals.


Common Arithmetic Operators

• Visual Basic provides operators for the common arithmetic operations:

+ Addition- Subtraction* Multiplication/ Division^ Exponentiation

Chapter 3- Slide 24


Common Arithmetic Operators

• AdditiondblTotal = dblPrice + dblTax

• SubtractiondblNetPrice = dblPrice – dblDiscount

• MultiplicationintArea = intLength * intWidth

• DivisiondblAverage = intTotal / intItems

• ExponentiationdblCube = dblSide ^ 3

Chapter 3- Slide 25

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